ABSTRACT
Of all the changes that take place in the physiology of a person acclimatizing to altitude, those resulting in an increase in ventilation are probably the most important. There are changes in both the hypoxic ventilatory response (HVR) and the hypercapnic ventilatory responses (HCVR). The changes in HVR are more difficult to measure but it is now accepted that HVR increases with time at altitude over a period of days to a few weeks. The changes in HCVR were characterized over 50 years ago and include a shift to the left and a steepening of the carbon dioxide response line. That is, a person when acclimatized responds to a lower Pco2 and is more sensitive to carbon dioxide than when unacclimatized. The time course of the changes in HCVR is exponential, with almost half taking place in the first 24 hours and most of the change being complete in about 2 weeks at a given altitude. These changes in the chemical control of breathing underlie the well-known increase in ventilation and result in a lower Pco2 and higher Po2 characteristic
of acclimatization. The mechanism underlying the increased carbon dioxide sensitivity may be due to the documented reduction in bicarbonate concentration in cerebrospinal fluid (CSF), blood and, presumably, brain extracellular fluid (ECF). The mechanisms of the increase in HVR are debatable but probably include changes in both the peripheral chemoreceptors and central processing of the signal in the brain.
